I am in the process of
rerouting mine on my 90. But I am basically rerouting everything almost
identically to the way it came on my 88 323 GTX.

The
hole in the front of the head gets Blocked off with a 30mm freeze plug and the
outlet to the rad with the thermostat is placed on the back of the head. I am
routing mine to the rad under the intake manifold instead of over the exhaust
manifold as others have done.

The
outlet to the Heater comes out the back of the head as well. To do this I welded
an area up on the intake manifold and added a hose fitting so it comes out of
the head through the intake manifold between cylinders 3 and 4. This is the same
as the GTX. A hose goes from here around the back of the engine and back up to
the heater connection at the firewall. It needs to be a bypass of the
thermostat.

The
Green connector on the bottom is the temperature sending unit that isnormally in the heater core outlet housing on the 1.6l Miata. The one
thatis on the radiator outlet housing that I moved to
the back of the engine isthe fan switch. I think both
of these are placed slightly different on the1.8l
versions and the 99+ head is different yet. I re-welded the boss for itto make it easier to get to the connector. It gets real close to thefirewall otherwise but it might fit. I am not sure if the M1 1.8l has thesame coolant ports hidden by the manifold or not but I am sure the 99+doesn't.

Some
other small hose connections are important. I welded two small hose nipples on
the steel hard-line that goes under the exhaust manifold.
One hose connects from the head just before the thermostat at the back to one of
the nipples. The Miata head already has a nipple for this hose that is capped
off with a rubber cap. This is important to keep a small amount of coolant
flowing across the thermal actuator of the thermostat. So it can sense what
temperature the rest of the engine is.

The other new nipple is for return coolant from the Turbo. The source coolant
for the turbo comes from the side of the block, same as 323 GTX. The Miata block
has a plug in this location sort of down by where the turbo oil source location
used by FM and the GTX is. Just need a small hose barb that threads in place of
the plug. I have one because I have a spare GTX engine.

The
small coolant return line from the intake manifold stuff just gets plugged into
the nipple on the front of the engine at the water pump inlet.

If
you want to see what the diagram will look like you can just look at a diagram
for a GTX and imagine rotating the engine 90deg while keeping the hoses
attacheed to the rad and the heater core. This is the way it was originaly
designed and it seems to make sense, definately works well on my GTX. I might be
able to email some pictures. I have a GTX shop manual.

I have everything ready for my to install and I plan to start this weekend.

>Do you mean under the
thermostat at the Miata stock location or the 323 stock location?

The small nipple I was referring to is on the back of
both heads. It points towards the intake side of the engine and is right on the
boss for the Thermostat housing. The GTX plumbs it back into the hard line
returning to the water pump from the heater core. On the stock Miata it is still
there but it is capped off.

Looking at my 90 Miata head and an 88 gtx head the only difference I can see is
that the Miata head has additional machining in the front so the thermostat
housing pedestal can be mounted to it. If you take the pedestal off the Miata
there is a machined bore that accepts a 30mm freeze plug which the GTX has at
that location. The blocks are Identical as far as I can tell.

I have not had the chance to look closely at any of the 1.8ís BP/BPT to see how
they might be different or even how they plumb the sandwich cooler but they were
used in the same all wheel drive configurations as the GTX so Iím guessing they
might be the same or very similar. The 99+ M2 heads might be totally different
however.

Well I don't have a 1.8l head to look at but I do know
there a littledifferent than my 1.6. The US never got
the 90-91 1.8l GTX so I haven't seenthat either. We
did have a 1.8l Ford Escort GT that had the same engine andhead design as the 1.8l Miata and it mounted the engine transversely. So
I'mfairly certain there is a way to do it very
similar to what I did. Itsgoing to be a little
different though. The way I did it has all routings thesame as the 1.6l B6T GTX. My set up seems to work great now. Before I
didit I had overheated on a track day and the
temperature would even rise wiledriving up mountain
roads. I have done two track days since and the
temperature stays rock solid. The engine and the heater still come up tooperating temperature as it should. I am running an FMII at 15psi and Idrive the car as a daily driver as well as autocrosses and open track
daysso I still have air conditioning hooked up as
well.

It
looks like there is a freeze plug between 2 and 3 from picturesI have found not sure if you could use that or not. It also looks like
theback of the head looks much different as well,
might make it really hard togo under the intake
manifold like I did. I am not sure what the plumbing
looks like for the oil filter sandwich cooler some of the 1.8l's have. Itmight be possible to tap into that somehow. I am not sure that the 99+
headwas used in any car other than the Miata so it
may have eliminated some ofthe options for coolant
routing as a cost savings.

I think getting the heater source from the front of the head as others havedone will work but its not Ideal. I have heard Richard M's kit might pullcoolant for the heater core from the back of the head somehow but I have
yetto see pictures of how that's actually done.
Pictures I have seen have justswapped things from the
front to the back and back to the front.

Something seems really wrong with the way this one is done to me.ScottIngram_coolant_reroute.htm
The source for the Heater core circuit comes from the upper rad hose afterthe thermostat. with the thermostat closed before the engine gets hot itwill try to suck coolant from the top of the radiator. Since the radiator
isbeing sucked on from the bottom at the same time by
the water pump I don'tthink their will be much if any
flow through the heater core or the rest ofthe engine
for that mater until the thermostat opens. And even when thethermostat does open the heater core circuit will serve as a bypass to
theradiator in parallel rather than in series as the
author seems to indicate.I bet you don't get
your heater to heat your car until the engine gets hot
enough to open the thermostat this way. And I bet there is really spiky andunstable temperature fluctuations with the thermostat opening and
closing.Might be fine for a track only car but
I think if that were the case thenjust remove the
heater core entirely.